Phorbol diester binding sites appear stable biologically, show no defect in number, affinity for H-3-labeled phorbol-12, 13-dibutyrate (PDBu) or down modulation in preneoplastic cells selected for resistance to 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced mitogenesis or promotion of transformation. Nor does malignant transformation detectably perturb these binding sites in mouse or human cells. Loss of epidermal growth factor (EGF) receptors correlates with loss of mitogenic response to TPA in monolayer culture, but has no effect on H-3-PDBu binding or TPA promotion of transformation. The roles of reactive oxygen production and divalent cation transport are being probed as possible mediators of signal transduction following receptor binding. Benzoyl peroxide, when applied to cells in liposomes during a period of 2 to 4 days, induces anchorage-independent transformation. This is inhibited by trisialoganglioside (GT), but not by retinoic acid, thus suggesting at least one difference between the promotion pathways for TPA and benzoyl peroxide. DNA strand breaks may be required for promotion of transformation of JB6 cells by benzoyl peroxide but are not required for promotion by TPA.